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Type:
Improvement
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Resolution: Done
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Priority:
Minor - P4
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None
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Affects Version/s: None
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Component/s: Aggregation Framework
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Labels:
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Query
Many accumulators ignore null-ish values, but $first/$last do not.
Observed behavior: If the first or last entry for a field is undefined, the $first / $last accumulator becomes undefined (the field is dropped).
Expected behavior: Potentially the first / last non undefined field could be chosen.
Test:
c = db.c;
c.drop();
c.save( {} );
c.save( { a:1 } );
c.save( {} );
// The 'a' field of the first document is undefined, so no 'z' result.
printjson( c.aggregate( { $group:{ _id:0, z:{ $first:'$a' } } } ) );
// The 'a' field of the last document is undefined, so no 'z' result.
printjson( c.aggregate( { $group:{ _id:0, z:{ $last:'$a' } } } ) );
// The 'z' result is [1], the undefined values of the first and last documents are excluded.
printjson( c.aggregate( { $group:{ _id:0, z:{ $push:'$a' } } } ) );
- related to
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- Closed
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